The invention relates to a control for dividing the output flow in hydraulic systems made available by at least one pump to a plurality of users according to the load pressure-independent principle given an output flow undersupply of the users, in particular, for construction machinery such as wheeled loaders, graders, excavators or the like.
For economical and technical-reasons it is often necessary to dimension hydraulic systems such that the output flow of the pump (or pumps), when the flow is fed simultaneously to two or more users, is no longer sufficient for supplying each user with the desired design output flow. This state is defined as undersupply of the individual users (e.g. cylinders) and is also present, for example, in the design of construction machinery, in particular, wheeled loaders, graders, excavators or the like, in which the selection of the pump size is such that the output flow can no longer guarantee the required adjustment times of the cylinders that are or can be actuated simultaneously, as can be demonstrated, for example, in the hoisting gear and tipping gear of a wheeled loader.
Given such an undersupply, a decision must be made when designing the hydraulic system regarding the extent to which the available output flow is distributed to the individual users. Hydraulic systems with control slide valves based on the throttle principle are not capable of dividing an output flow when the slide valve piston is switched through, because, already at a very small pressure difference between the users, the entire output flow flows to the user with the lesser load.
The load pressure-independent (load sensing) system, on the other hand, offers the possibility of dividing the output flow to the individual users according to certain criteria. In order to obtain, under the given conditions of the respective application, the most favorable motional flow of the components actuated by the hydraulic users (e.g., cylinders) such as, e.g., lifting frames, booms, etc., it is necessary to divide the output flow according to a certain ratio.
To date, solutions are known in which, depending on the embodiment, a division is provided which is fixed or which depends on outside conditions. This applies, inter alia, for the following embodiments:
Pressure scales connected in parallel in the feed to each piston valve (0+P Ohydraulik und Pneumatik [O+P Oil Hydraulics and Pneumatics], 35 (1991) No. 9, pages 717 to 723--Regelung hydraulischer Antriebe mit veranderlichem Versorgungsdruck [Control for Hydraulic Drives with Variable Supply Pressure]). In case of undersupply, the user with the lower pressure requirement is fed an increasing higher portion of the output flow when the differential pressure between the users increases. Given a 50% degree of supply of two users having the same nominal through-flow rate, the user with the lower pressure load already receives the entire output flow when the pressure difference compared to the user with the higher load corresponds to the control pressure of the pressure scale. Since the control pressure of the pressure scale is in the magnitude of 5-20 bar, the entire output flow will therefore be respectively fed to another user at low changing pressure differentials.
Priority pressure scales may also be connected in series in the feed to each control slide valve. When two or more users are actuated simultaneously, the user which is disposed closer to the pump respectively receives the desired volume in the course of the pump channel, while the remaining stream is offered to the further users. Therefore, given a 50% degree of supply of two users having the same nominal through-flow rate, the user which is disposed closest to the pump connection in the slide valve receives the entire output flow (Off Highway Capability--Wheeled Loaders of the Vickers Company).
Load pressure-independent through-flow distribution (LITD) due to a compensator connected downstream of every control slide valve, with the compensator system known to date consisting essentially of a piston and spring. Here, each user receives a proportionally lower output flow depending on the degree of undersupply. Thus, given a degree of supply of 50% and two users, each receives 50% of its nominal rate, regardless of whether the users are designed for identical or different nominal through-flow rates (Mannesmann Rexroth Catalog--RD 64291/03.92, pages 163, 164 and 166--load sensing control block M7 series with LITD in mixed construction Mono+Sandwich).